For long spans it is usual to use a suspended structure in which the weight is carried by cables extending between towers at the ends of the main span or spans and the deck itself is primarily designed to give stiffness rather than strength. Similar considerations apply to cable-stayed structures in which cables for supporting the deck are connected directly between the deck and supporting towers at the end of the span. In these designs, and indeed in any bridge design in which the deck is not part of a substantially rigid structure but is free to twist about its longitudinal axis, it has been known for many years that with high winds transverse to the span, aerodynamically-induced instability could arise. This instability might be "flutter", that is to say torsional oscillations of the deck which increase with time, or "divergence", that is a twist deflection which increases exponentially. In either case distortion of the bridge could occur.
To minimize the danger of such instability occuring, or to raise the wind speed at which it will occur above the maximum which can be expected at the site of the bridge, it has been usual to provide extra torsional stiffness in the deck. Stiffening by means of vertical girders at the edges of the deck is not usually sufficient and has therefore been supplemeted by a transverse truss below the deck. In more recent designs the stiffening has been effected by a streamlined steel torsion box of which the upper surface carries the traffic. It has also been proposed in U.K. Patent Specification No. 1,523,811 to reduce the aerodynamic effects by perforating or slotting the deck, thereby enabling it to be supported at the centre of transverse beams which are suspended from cables more widely spaced than normal for the width of the deck to increase the torsional stiffness.